Rivets are probably the oldest and most widely used fasteners in the industry. They are inexpensive to manufacture, do not require close tolerance holes, and they are easily installed. Riveted structures usually have an extended service (fatigue) life because of the hole-filling characteristics of the rivet. When rivets are driven, the rivet shank expands and tends to fill the hole. This eliminates relative movement between the joined structural members, thus, providing the extended fatigue life.
Pins installed with an interference fit, which puts residual hoop tension stresses around the hole, are used to enhance the fatigue life of structures. These pins are expensive and require very close tolerance holes and, therefore, are expensive to install. One such fastener is the Hi-Tigue pin, manufactured by the Hi-Shear Corporation, Torrance, Calif.
It would be more desirable to place residual hoop compression stresses around the hole. One technique to accomplish this, which has been developed by the Fatigue Technology Corporation, Seattle, Wash., is to expand the hole with a removable sleeve and mandrel. The mandrel, while it is being pulled through the hole, stresses the material around the hole beyond the proportional limit in hoop tension. Thus, when the mandrel is removed, the material around the hole recovers elastically and forms residual hoop compression locally around the hole.
To obtain better control of the filling of the hole when driving, various rivet designs have been proposed and developed. For example, Russian Pat. No. 516,848 discloses a generally flush-head rivet wherein the countersunk angle in the material being joined is less than the included underside taper of the head. Here, again, this is an attempt to insure that initially all the material in the head is driven into the hole in the structure being joined. Note that in the Russian patent, excess material is provided in the head. While the patent does discuss equal deformation of the holes running through the plates being joined, there appears to be no disclosure of expanding the holes to improve fatigue life.
The concept of providing excess material in the head of a flush-head rivet is also disclosed in U.S. Pat. No. 3,526,032, "Riveting Method Employing Metal Flow in Both the Manufactured Head and the Upset Head" by F. C. Pipher. Here a spherical surface is provided on the top of the flush-head rivet. The ratio of height of the domed portion to the nominal diameter of the rivet is between 0.039 and 0.048. But this rivet is designed for use in a hole where the countersunk angle is equal to the included angle of the underside of the head of the rivet. This rivet is known to provide improved filling of the hole when compared to commercially available NAS1097 or MS20426 rivets but does not provide significant expansion of the hole to improve fatigue life.
In none of the above rivets has an attempt been made to make all critical dimensions of the head and tail, before and after driving, of the rivet a function of the shank diameter. Requiring that the critical dimensions be a ratio of the shank diameter makes the design of any rivet with a particular shank diameter easy to accomplish. While in most cases, standard rivet sizes will suffice, there are instances where special diameters (inch to metric conversions, etc.) are required. Thus, a rivet having all the critical dimensions based on the shank diameter would minimize design and test time required to insure that the necessary static tension, shear strengths, and fatigue life are achieved.
Material selection for rivets is also a critical problem. If the material has a very high-yield stress, there will be difficulty in driving the rivet because of the tendency of the bucked tail portion of the rivet to crack and fatigue life will be decreased due to a lack of hole filling. If the material is too malleable, the rivet may not have sufficient strength. So the selection of material properties becomes a trade-off.
One of the ways around this problem has been the use of a 2024-T4 aluminum alloy (icebox) rivet which, when solution heat-treated, quenched, and stored at a temperature below zero degrees Fahrenheit, will remain in a relatively soft condition. Upon return to room temperature, the aluminum alloy rivet must be driven within 15 minutes since it quickly age hardens. Thus, the name "icebox" rivet. While this rivet is widely used, it is expensive to handle. If prematurely brought to room temperature, it cannot be driven without cracking the bucked tail. Rivets inadvertently allowed to harden prior to driving must be reheat-treated. Therefore, from the time of removal from the refrigerator and transport to the assembly station, the rivets are carried and stored in a container filled with dry ice. It is inevitable that some rivets will be driven after they are too hard; thus, they crack and must be drilled out and replaced. The added expense is obvious. Therefore, a considerable cost savings can be achieved, particularly in the manufacture of aircraft, if a non-icebox material that provides the same static strength and fatigue life as the icebox rivet can be obtained.
Additional patents of interest are as follows: U.S. Pat. No. 221,447, "Rivet" by J. B. Cornell; U.S. Pat. No. 2,237,338, "Rivet" by H. S. Dale; U.S. Pat. No. 3,821,871, "Fatigue Resistant Fasteners" by H. A. Schmitt; U.S. Pat. No. 3,927,458, "Crowned Head Rivet Method" by E. R. Speakman; U.S. Pat. No. 4,004,484, "Crowned Flush Head Rivet" by E. R. Speakman; U.S. Pat. No. 4,051,592, "Expanding Head Riveting Method" by F. S. Briles; U.S. Pat. No. 4,230,016, "Fatigue Resistant Fasteners and Method of Manufacturing Joints Therewith" by H. B. Merrell and U.S. Pat. No. 4,245,921, "Continuously Curved Countersink used with Deformable Rivet" by J. G. Falcioni.
Therefore, it is a primary object to provide a rivet that, after driving, obtains controlled expansion of the hole, placing residual hoop compression stresses in the hole wall, thereby increasing the fatigue life.
It is another object of the subject invention to provide a rivet wherein all critical dimensions of the head and shank before and after driving thereof are ratios of the shank diameter.
It is another object of the subject invention to provide a 7050 aluminum alloy rivet that can be used directly as a substitute for 2024 aluminum alloy icebox rivet.
An additional object of the subject invention is to produce a rivet that can be installed and driven in holes produced with standard twist drills and 100.degree. countersinks.
A further object of the subject invention is to provide a rivet that can be driven with all standard flush rivet driving tools while still maintaining controlled expansion of the hole, placing the hole wall in compression, increasing fatigue life.
A still further object of the subject invention is to provide a rivet that can be installed and driven with or without tank sealant or primer.